If your looking for stiffness, try 7075 T6 Aluminum. It's much much stronger thatn 6061 T6, and a tad lighter as well.

I couldnt find any for a decent price....neither of the places my college orders from carry 7075 T6 in anything but 4x8 sheets and bigger, i can get the 6061 T6 in smaller pieces like 1x2 feet and its alot cheaper,

i do already have some .160 7075 T6 but thats WAY to thick and heavy. Its good for making skids plates for atvs though.

Too bad you're on a budge, because the CF from DragonPlate is amazingly tough... I bought their sample pack, and it's very well made.. just major $$$$

Polycarbonate. Easy to cut. Tough and scratch resistant (So some say).

This is where I buy all of my materials.

honestly i ended up getting the aluminum for free since the instructor said we need to have some for some brackets that needed made....he said i can cut out what i need and hey its hard to beat something thats free.

If this helps I purchased my Carbon Fiber from a company called outfrontdrames. The guys name is wes. Let him know what your looking for and he will hook you up. He has real high grade Carbon Fiber and uses it for RC projects he builds. If you go to my signature you will see my website. Go to the links area and look for his site. I think for your best bet you should look in the neighborhood of 3mm CF